1. Field of the Invention
The invention relates in general to packet-switched services in cellular networks. In particular, the invention relates to a packet-switched handover procedure for a mobile station changing cells.
2. Description of the Related Art
A communication system can be seen as a facility that enables communication sessions between two or more entities such as user equipment and/or other nodes associated with the communication system. The communication may comprise, for example, communication of voice, data, multimedia and so on. Communication systems providing wireless communication for user equipment are known. Cellular communication systems are configured to have a cell structure, and typically they support communication with user equipment changing locations (mobile users). The support for communications for mobile users may include support for handing existing connections from one cell to another cell. At least routing of calls or communications for a mobile user in a new cell is typically supported in cellular systems. Some examples of a cellular system are the Global System for Mobile Telecommunications (GSM) and General Packet Radio Service (GPRS). GPRS provides packet-switched data services and utilizes the infrastructure of a GSM system. Two further examples of cellular systems are EDGE and EGPRS, which are further enhancements to GSM and GPRS. EDGE refers to Enhanced Data Rates for GSM Evolution, and EGPRS refers to EDGE GPRS.
For illustrating packet-switched services in cellular system, GPRS and EGPRS systems are used below as examples. It is, however, appreciated, that similar concepts may be found also in other cellular systems supporting packet-switched services.
In the following description, reference is made to certain Third Generation Partnership Project (3GPP) technical specifications. These technical specifications are known to a person skilled in the art of cellular networks.
FIG. 1 illustrates schematically, as an example of a cellular network supporting packet-switched services, a GSM/GPRS network 100. Alternatively, the system 100 may be an EDGE/EGPRS network. Only some of the network elements of a GSM/GPRS network are illustrated in FIG. 1. The radio access network 110 comprises a number of base station systems (BSS) 112a, 112b. Each base station system 112 comprises a base station controller (BSC) 114 and a number of base stations (BS) 116. A mobile station (MS) 101 communicates with a base station 116 over a radio interface. The packet-switched core network 120 of the system 100 comprises a number of GPRS Supporting Nodes (GSN) 122. Each mobile station registered for packet-switched services has a serving GSN, called SGSN, which is responsible for controlling the packet-switched connections to and from the mobile station. The packet-switched core network 120 is typically connected to further packet-switched networks via a Gateway GSN (GGSN).
FIG. 2 shows schematically the protocol stacks of some of the network elements illustrated in FIG. 1 and identifies some interfaces. The interface between an SGSN and a BSS is called Gb. In the SGSN protocol stack and in the BSS protocol stack towards the SGSN the following protocols are common. The lowest protocol is called Layer 1. The second protocol is Network Service (NS), and the third protocol is Base Station System GPRS Protocol (BSSGP). The fourth protocol in the SGSN protocol stack in Link Layer Control (LLC), and the counterpart for this protocol entity is found in the MS protocol stack.
The interface between a MS and a BSS is called Um. The protocols are common in the MS protocol stack and in the BSS protocol stack towards the MS are the following: the lowest protocol is called the physical layer (PHY), the second in the Media Access Control (MAC) protocol and the third in the Radio Link Control (RLC) protocol. In the MS protocol stack, there is further the LLC protocol and on top of that further protocols or applications. In the BSS protocol stack, data is relayed between the RLC protocol and the BSSGP protocol.
A GPRS or EGPRS network assigns a temporary identifier for a mobile station wishing to have access to packet-switched service. This identifier is a Packet-Temporary Mobile Subscriber Identifier (P-TMSI), and it is assigned by the SGSN. P-TMSI handling is discussed in TS 23.003 and TS 24.008. A further identifier, a Temporary Logical Link Identifier (TLLI), is used for addressing resources allocated for GPRS services at RLC/MAC layer on the Um interface and in the BSSGP layer on the Gb interface. The value for TLLI is built by the MS or by SGSN either on the basis of the Packet-Temporary Mobile Subscriber Identity (P-TMSI) or directly (random TLLI). TLLI handling is discussed in TS 23.003.
A packet data protocol (PDP) context refers to information sets held in MS and GPRS Supporting Nodes (GSNs) that are used to bind the MS to an PDP address that identifies an application, PDP type and a QoS profile. PDP context functions are discussed in 3GPP TS29.060.
For identifying MS PDP contexts, the TLLI identifier is used together with a Network layer Service Access Point Identifier (N-SAPI). N-SAPI is an identifier used at a Subnetwork Dependent Convergence Protocol (SNDCP) layer in a mobile station and in a SGSN.
A further identifier for identifying packet-switched services is the Packet Flow Identifier (PFI). The PFI identifier is assigned by the SGSN, and the PFI identifies a packet flow for a certain MS. A mobile station may have more than one packet flow.
Information related to quality of service characteristics of the user data transmission in GPRS/EGPRS for a specific packet flow is kept in a BSS Packet Flow Context (BSS PFC). A BSS PFC thus relates to one packet flow identified by a PFI. The BSS PFC is given to a BSS by the SGSN. BSS packet flow contexts related to one MS are stored in an MS specific BSS context identified by TLLI. Within the BSS context, a BSS PFC is identified by the packet flow identifier PFI.
A Temporary Block Flow (TBF) is allocated radio resources on one or more packet data channels (PDCHs) that comprise a number of RLC/MAC blocks carrying one or more upper layer PDUs. A TBF is temporary and is maintained only for the duration of the data transfer. A TBF may operate in either GPRS or EGPRS TBF mode. Radio Resources allocated for an MS are addressed by TLLI.
In the following, resources allocated for packet-data services in a GSM/EDGE Radio Access Network (GERAN) are discussed. Depending on which interface is used to connect the radio access network to the core network, there are two types of GERAN architectures: GERAN A/Gb mode and GERAN lu mode. The following description relates in particular to GERAN A/Gb mode, thus it may not be applicable to GERAN lu mode. There are two main identifiers for packet switched service classes and mobile stations in GERAN A/Gb mode: the Packet Flow Identifier (PFI) and the Temporary Logical Link Identity (TLLI).
A cell refers to a basic unit of the cellular network. Each base station may form a cell, or base station may be provided with transceivers, whereof each forms a cell. In a GPRS/EGPRS network, a mobile station 101 is communicating with one base station (cell) at a time. When the mobile station 101 moves, it changes cell. In a GSM network, term handover refers to handing over (circuit-switched) connections from the old (source) cell to a new (target) cell.
There is need also for a handover for the packet-switched connections. A packet-switched handover would minimize the service interruption times by allowing continuous data transfer between a MS and a cellular system, when the MS is moving from one cell to another cell. A packet-switched handover may be an intra-SGSN handover or an inter-SGSN handover. In an intra-SGSN handover, the source and target BSS are controlled by the same SGSN. In an inter-SGSN handover, the source BSS is controlled by a first (source) SGSN and the target BSS is controlled by a second (target) SGSN.
There exists a proposal in the 3GPP TSG GERAN, namely “Support of Conversational Services over the PS domain; Technical Report (Release 6)” version 0.8.0, for a packet-switched handover procedure in a GPRS/EGPRS network. This proposal provides the technical solutions to support conversational QoS class in the GERAN A/Gb mode. Conversational QoS class is used to carry real-time traffic flows most sensitive to delay. In this proposal, a packet-switched handover for conversational QoS class comprises two phases: a preparation phase and an execution phase.
FIG. 3 schematically illustrates, as an example of a packet-switched inter-SGSN handover, the preparation phase of an inter-SGSN PS handover in accordance with the above mentioned proposal. In phase 301, a source BSS makes a decision to perform an A/Gb PS Handover. The source BSS then sends a PS Handover Required message 302 to the source SGSN. This message 302 contains various identifiers, including a target cell identifier. Based on the target cell identifier, the source SGSN determines the target SGSN, and thereafter informs the target SGSN about the handover with Prepare PS Handover Request message 303. Upon receipt of this message, the target SGSN assigns a P-TMSI value for the mobile station in phase 304. Then the target SGSN send a PS Handover Request message 305 to the target BSS. In phase 306 radio resources are allocated in the target BSS for the packet-switched services relating to the mobile station. After radio resource allocation, the target BSS sends a PS Handover Request Acknowledgement 307 to the target SGSN. The target SGSN sends then a Prepare PS Handover Response message 308 to the source SGSN. In phase 309, the source SGSN starts bi-casting packet data to the target SGSN.
In the beginning of the execution phase of the proposed packet-switched handover, the source SGSN sends a PS Handover Command message to the source BSS. The source BSS, in turn, sends PS Handover Command message to the mobile station. The P-TMSI value assigned by the target SGSN is delivered to the mobile station in this PS Handover Command message. The mobile station changes cells from the source cell to the target cell, and after certain procedures relating to the cell change The execution phase continues with a PS Handover Complete Message sent by the mobile station to the target BSS. The target BSS sends a PS Handover Complete message to the target SGSN. Thereafter the target SGSN and the GGSN update PDP context with Update PDP Context Request and Update PDP Context Response messages. After the PDP context update in the GGSN, the target SGSN sends a Forward PS Handover Complete message to the source SGSN. The source SGSN responds with a Forward PS Handover Complete Acknowledgement message. Thereafter BSS packet flow procedures are carried out between the source SGSN and the source BSS. In the end of the execution phase, a routing area update (RAU) procedure is carried out. The mobile station initiates this routing area update by sending a RAU Request message.
In the above discussed inter-SGSN packet-switched handover (FIG. 3), a target SGSN needs to assign a P-TMSI for the MS for the target cell before the MS is residing in the target cell. In addition to the above mentioned example, a SGSN may need to assign a new P-TMSI for a MS also in an intra-SGSN handover. This is the case, for example, when the source cell and the target cell belong to different routing areas (RA).
As mentioned above, the proposed packet-switched handover procedure is a quite a complex procedure having a preparation phase and an execution phase. A set of messages relating to packet-switched handover is defined, this set comprising at least nine new messages: PS Handover Required, Prepare PS handover Request, PS Handover Request, PS Handover Request Acknowledge, Prepare PS Handover Response, PS Handover Command, PS Handover Complete, Forward PS Handover Complete, and Forwards PS Handover Complete Acknowledge. It should be also noted that failure scenarios may become quite cumbersome with such a handover procedure having a preparation phase and an execution phase and a significant number of signaling messages between various network elements.
Furthermore, there may be need for some changes also in the Routing Area Update procedures, as the BSS PFC (Packet Flow Context), MM, and PDP contexts need to be exchanged during the packet-switched handover procedure.
Furthermore, the mobile station should also be able to cope with possible failures in the handover procedure. An example of such a possible failure is that access in the target cell fails and the MS returns to the source cell.
One aim of the embodiments of the present invention is to present a straightforward procedure for a packet-switched handover.